1. Field of the Invention
The present invention relates to quantum information processing, and, in particular, to techniques for producing, storing, and retrieving quantum bits represented by single photons.
2. Description of the Related Art
The past approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not to be considered prior art to the claims in this application merely due to the presence of these approaches in this background section.
Information processing using classical computers relies on physical phenomena, such as magnetic fields, voltages, and optical intensity that can be produced and measured in each of two base states, one base state representing a zero and another base state representing a one. Each physical element that can achieve either of these two states represents one binary digit, called a bit. Quantum information processing uses physical elements that exhibit quantum properties that may include, not only one of the two or more base states, but also an arbitrary superposition state of the base states. A superposition state has some non-zero probability of being measured as one of the base states and some non-zero probability of being measured as another of the base states. A physical element that exhibits quantum properties for two base states represents one quantum bit, also called a qubit. Physical elements that are suitable for representing qubits include the spins of single electrons, electron states in atoms or molecules, nuclear spins in molecules and solids, magnetic flux, spatial propagation modes of single photons, and polarizations of single photons.
Logical operations performed on qubits apply not only to the base states of those qubits but also to the superposition states of those qubits, simultaneously. Quantum computers based on logical operations on systems of qubits offer the promise of massively simultaneous processing (also called massively parallel processing) that can address problems that are considered intractable with classical information processing. Such classically intractable problems that can be addressed with quantum computers include simulation of quantum interactions, combinatorial searches in unsorted data, finding prime factors of large integers, solving for cryptographic keys used in current secure communication algorithms, and truly secure communications (also called “quantum cryptography”).
Obstacles to achieving quantum computers include the difficulty in isolating qubits from uncontrolled interactions with the environment, and transmitting qubits. Many of the physical elements that represent qubits, such as molecules and solids, are not readily transmitted, and interact strongly with their environment.
Single photons, however, interact little in many environments, including glass fiber and air, and are easily transmitted in such media. Therefore several approaches have utilized quantum properties of single photons.
One approach implements logical operations on single photons using non-linear interactions between single photons. A problem with non-linear interactions between single photons is that such interactions are very weak and no devices satisfactorily implement this approach.
Another approach uses linear interactions between single photons but relies on interferometer techniques, e.g., interference on two spatial modes of propagation for a single photon. For example, logic gates using this approach have been proposed by E. Knill, R. Laflamme, and G. J. Milburn, “A scheme for efficient quantum computation with linear optics,” Nature, vol. 409, p. 49, 4 January 2001 (hereinafter Knill) and by M. Koashi, T. Yamamoto, and N. Imoto, “Probabilistic manipulation of entangled photons,” Physical Review A, vol. 63, 030301, 12 Feb. 2001 (hereinafter Koashi). These devices are called “probabilistic” logical gates because they perform the desired logical operation in response to only a fraction of the input photons. However, it can be determined when an operation is performed successfully, so that, in a separate step often called a “post selection” step or a “post-detection selection” step, output photons are blocked unless the operation is successfully performed. It has been shown that the fraction can be increased close to a value of one with sufficient numbers of components and extra photons (called “ancilla”) in particular states.
Probabilistic, linear devices proposed by Knill suffer from errors due to thermally induced phase shifts on the two spatial modes. Other probabilistic, linear devices proposed by Koashi reduce the phase shifts by including a large number of additional components and other resources, such as sources of a large number of qubits in particular states. Pittman I disclosed devices that perform logical operations on quantum states of single photons that operate on the polarization states of single photons and that do not suffer thermally induced phase shifts and that do not require a large number of additional components and resources. The Pittman I devices do employ some ancilla.
Many of these approaches for quantum computing benefit from a reliable source of single photons on demand in a particular state, with particular temporal and spectral properties, for example, to serve as a simple source for the ancilla described above, or to demonstrate the operation of the quantum logic operations. Furthermore, practical quantum computing is expected to utilize some mechanism for storing quantum bits with arbitrary values, for example to temporarily hold intermediate values during an extended computation.
Some approaches for providing single photons rely on a spontaneous emission of an isolated two-state quantum system, such as a single atom, ion, or quantum dot. However, these approaches for providing single photons suffer from at least two deficiencies: 1) uncertainty about whether a photon has been emitted; and 2) uncertainty about its direction. Although the probability of single-photon emission can be high in these approaches, there is no extant method for ensuring that a photon has actually been emitted. Furthermore, in some approaches, when a single photon is emitted, its direction may be any in a solid angle 4□ encompassing all directions.
Based on the foregoing there is a clear need for a source of single photons in a specified state that provides a certain emission of a photon and with a known direction.
Existing approaches for storing qubits exploit the persistent nature of some phenomena that represent qubits. For example, in an ion-trap approach to quantum computing, the qubits are stored in potentially long-lived atomic states. However, these approaches are not directly applicable to qubits represented by the states of single photons, and no method currently exists for effectively converting and storing the arbitrary states of single photons in the long-lived atomic states and then retrieving and converting back to the original states of single photons.
Challenges in providing a memory for photonic qubits in arbitrary states include that: 1) the single photon qubit to be stored travels at the speed of light in the storage medium; 2) the single photon qubit must be stored so as to maintain its arbitrary quantum state (the coherence of the stored photon); 3) the single photon must be retrieved on demand; and 4) the single photon qubit must be stored and retrieved without measuring the state of the photon. This fourth challenge arises because a measurement results in one of the base states and destroys the superposition state of the photon, e.g., eliminates the probability that the photon will be measured in a different base state.
Based on the foregoing description, there is a clear need for techniques for a quantum memory to store and retrieve qubits represented by arbitrary polarization states of single photons.